build: switch to yarn

Author: TheAlexLichter

.travis.yml

@@ -1,15 +1,15 @@
 language: node_js
 node_js:
-  - "8"
-  - "9"
-  - "10"
+- "8"
+- "9"
+- "10"
 cache:
-  npm: true
+  yarn: true
   directories:
-    - node_modules
+  - node_modules
 install:
-  - npm install
+- yarn
 script:
-  - npm t
+- yarn test
 after_success:
-- npm run coverage
+- yarn coverage